This invention relates to preparing extruded articles having surface microstructures formed with a microcellular polymer foam. Generally, a polymer foam includes a polymer matrix and is characterized by a density that is lower than the density of the polymer matrix itself. Density reduction is achieved in a number of ways, including through creation of gas-filled voids in the matrix (e.g., by means of a blowing agent). The foam void is of a size less than that of the microstructures.
In order to improve the mechanical properties of standard cellular foamed materials, a microcellular process was developed for manufacturing foamed plastics having greater cell densities and smaller cell sizes. Such a process is described, for example, in U.S. Pat. No. 4,473,665. The process presaturates the plastic material with a uniform concentration of a gas under pressure. A sudden induction of thermodynamic instability then nucleates a large number of cells. For example, the material is presaturated with the gas and maintained under pressure at its glass transition temperature. The material is suddenly exposed to a low pressure to nucleate cells and promote cell growth to a desired size, depending on the desired final density, thereby producing a foamed material having microcellular voids, or cells, therein. The material is then quickly further cooled, or quenched, to maintain the microcellular structure. Such a technique tends to increase the cell density, i.e., the number of cells per unit volume of the parent material, and to produce much smaller cell sizes than those in standard cellular structures. The resulting microcellular foamed materials that are produced, using various thermoplastics and thermosetting plastics, tend to have average cell sizes in the range of 3 to 10 microns, with void fractions of up to 50% of the total volume and maximum cell densities of about one billion voids per cubic centimeter of the parent material.
Microcellular foamed plastic materials are also described in U.S. Pat. No. 4,761,256 which describes a web of plastic material impregnated with an inert gas. The web is reheated at a foaming station to induce foaming, the temperature and duration of the foaming process being controlled prior to the generation of the web to produce the desired characteristics. The process is designed to provide for production of foamed plastic web materials in a continuous manner. The cell sizes in the foamed material is stated to be within a range of from 2 to 9 microns in diameter.
U.S. Pat. No. 5,334,359 describes foamed materials which can be of smaller cell sizes, e.g., 1.0 micron or less. The materials also allegedly have a wide range of void fraction percentages from very high void fractions (low material densities) up to 90%, or more, to very low void fractions (high material densities) down to 20%, or less.